On 10/13/21 04:32, Xiaoguang Wang wrote:
hi,
On 10/12/21 14:11, Xiaoguang Wang wrote:
On 10/12/21 09:48, Xiaoguang Wang wrote:
The idea behind register file feature is good and straightforward, but
there is a very big issue that it's hard to use for user apps. User apps
need to bind slot info to file descriptor. For example, user app wants
to register a file, then it first needs to find a free slot in register
file infrastructure, that means user app needs to maintain slot info in
userspace, which is a obvious burden for userspace developers.
Slot allocation is specifically entirely given away to the userspace,
the userspace has more info and can use it more efficiently, e.g.
if there is only a small managed set of registered files they can
always have O(1) slot "lookup", and a couple of more use cases.
Can you explain more what is slot "lookup", thanks. For me, it seems that
I referred to nothing particular, just a way userspace finds a new index,
can be round robin or "index==fd".
use fd as slot is the simplest and most efficient way, user does not need to> mange slot info at all in userspace.
As mentioned, it should be slightly more efficient to have a small table,
cache misses. Also, it's allocated with kvcalloc() so if it can't be
allocate physically contig memory it will set up virtual memory.
So, if the userspace has some other way of indexing files, small tables
are preferred. For instance if it operates with 1-2 files, or stores files
in an array and the index in the array may serve the purpose, or any other
way. Also, additional memory for those who care.
Yeah, I agree with you that for small tables, current implementation seems good,
If user app just registers a small number of files, it may handle it well, but imagine
how netty, nginx or other network apps which will open thousands of socket files,
manage these socket files' slot info will be a obvious burden to developer, these
apps may need to develop a private component to record used or free slot. Especially
in a high concurrency scenario, frequent sockes opened or closed, this private component
may need locks to protect, that means this private component will introduce overhead too.
For a fd, vfs layer has already ensure its unique.
If userspace wants to mimic a fdtable into io_uring's registered table,
it's possible to do as is and without extra fdtable tracking
fd = open();
io_uring_update_slot(off=fd, fd=fd);
No, currently it's hard to do above work, unless we register a big number of files initially.
If they intend to use a big number of files that's the way to go. They
can unregister/register if needed, usual grow factor=2 should make
it workable.
I'm not sure un-register/register are appropriate，say a app registers 1000 files, then
it needs to un-register 1000 files firstly, there are doubts whether can do this un-registration
work, if some of these files are used by other threads, which submit sqes with FIXED_FILE
flags continually, so the first un-registration work needs to synchronize with threads which
are submitting requests. And later app needs to prepare a new files array, saving current 1000
files and new files info to this new array, for me, it can works, but not efficient and somewhat
hard to use :)
Sounds reasonable. What I oppose is wiring it solely based on fd. On the
other hand, it sounds what you need is a "grow table" feature.
We can also think about adding new format, instead of array of fds, add
passing an array of pairs {offset, fd}.
--
Pavel Begunkov
